It is known to use image rejection techniques in a radio receiver by the use of high selectivity in a tuned circuit prior to and subsequent to the RF amplifier circuit. However, more recently there has been an increased demand for a broadly tuned RF preselector to pass without distortion amplitude and phase modulated signal information. Under these conditions it is desirable to reject image or other undesired frequencies of the desired RF signal.
Various approaches used to accomplish image frequency rejection have been to tune the RF signals in isolated circuits as described, for example, in U.S. Pat. No. 3,528,023 to Jeffers. Still another approach has been to receive the RF signal and amplify it and use this output to cooperate with an oscillator to mix with the RF to produce an IF as described for example in U.S. Pat. No. 3,348,155 to Von Recklinghausen.
In accordance with the present invention an RF amplifying system having image frequency rejection is provided by making the gain of the RF amplifier frequency dependent by the use of an anti-resonant circuit tuned to the image frequency causing degeneration of the normal amplifying function at the specified image frequency.